1. Field of the Invention
The present invention relates to a control device for a linear oxygen sensor.
In particular, the present invention relates to a control device for a linear oxygen sensor known as a xe2x80x9cUEGOxe2x80x9d sensor (Universal Exhaust Gas Oxygen sensor), to which the following description will make explicit reference without thereby losing its general applicability.
The present invention is advantageously applicable in the field of motor vehicle manufacturing, in which there is a known use of a UEGO sensor located in the exhaust pipe of an internal combustion engine to obtain information on the composition of the exhaust gases.
2. Description of the Related Art
The UEGO sensor has two electrolytic cells sensitive to oxygen ions, called respectively the xe2x80x9cpumping cell Ipxe2x80x9d and the xe2x80x9csensing cell Vsxe2x80x9d, and a diffusion chamber located between these cells and capable of receiving part of the combustion gases leaving the engine. The UEGO sensor also has a reference chamber which is capable of containing a specified percentage of oxygen, namely, for example, a percentage of oxygen equal to that which the exhaust gases would have if the air/fuel ratio of the mixture supplied to the engine were stoichiometric. Alternatively, the reference chamber could contain a percentage of oxygen equal to that contained in the atmosphere.
The UEGO sensor requires the use of a controller, which is connected to the sensor by means of a connector, and is capable of controlling the current to the pumping cell Ip to exert a feedback control action on the sensor. In particular, the controller, on the basis of the difference between the percentage of oxygen present in the exhaust gases inside the diffusion chamber and the percentage of oxygen present in the reference chamber, regulates the current supplied to the pumping cell Ip in order to generate a mechanism for draining oxygen ions from the diffusion chamber to the external environment (or vice versa). This draining mechanism has the function of modifying the percentage of oxygen present in the diffusion chamber in such a way that the ratio between the percentages of oxygen in the diffusion chamber and in the reference chamber takes a specified value. This means that this draining mechanism has the role of adjusting the percentage of oxygen in the diffusion chamber to a specified value, in order, for example, to maintain the diffusion chamber in a state of stoichiometry.
The intensity of the control action, in other words the strength of the current supplied to the pumping cell Ip to maintain the stoichiometry in the diffusion chamber, is the information according to which the controller generates an output signal representing the ratio of the exhaust gases leaving the engine. This output signal, as is known, is used by the engine control unit, for example in order to correct the quantity of fuel to be supplied to the cylinders.
Two types of UEGO sensors are at present available on the market, and differ principally in the way in which the quantity of oxygen desired in the reference chamber is generated. In particular, one of the two types of UEGO sensor receives oxygen in the reference chamber directly from the external environment through the electrical cables which connect it to the connector, while the other type of UEGO sensor generates the oxygen itself in the reference chamber. This is done by making a polarization circuit of the corresponding controller send a polarization current to the sensing cell Vs, to generate a mechanism for draining oxygen from the exhaust gases in the diffusion chamber to the reference chamber.
In the present state of the art, both types of sensor have to be controlled by corresponding dedicated controllers, each of which has the limitation of not being capable of controlling a type of sensor different from that with which it is associated. This is because each of the two types of controller is physically constructed in such a way that it can only be connected to the type of sensor associated with it.
Moreover, each of the two types of control device has a drawback associated with the precision of the output signal.
This is because each controller of the known type has to be connected to a compensating resistance capable of compensating any losses of the current supplied to the pumping cell Ip, before the controller is installed in the vehicle. This compensating resistance, whose nominal value is indicated by the manufacturer of the sensor on completion of manufacture, is connected between two terminals of the connector, and, by interacting with the controller, intervenes actively in the generation of the output signal. In particular, the two terminals between which the compensating resistance is connected differ according to the type of sensor which is to be connected.
Unfortunately, the compensating resistance, being located in the engine compartment, is subjected to intense thermal stresses during the operation of the engine. Consequently, as the engine temperature varies and/or as a result of oxidation due to atmospheric agents, the compensating resistance may come to have values in relation to the controller which differ from the nominal value which ensures correct compensation. This means that the losses of driving current are not adequately compensated and the signal at the output of the controller does not accurately indicate the composition of the exhaust gases.
The object of the present invention is to provide a control device for a linear oxygen sensor, particularly a UEGO sensor, which overcomes the limitations of the known controllers, or in other words is capable of controlling both of the types of sensor mentioned above.
According to the present invention, a control device is provided for a linear oxygen sensor capable of being located in an exhaust pipe of an internal combustion engine, the control device comprising a controller of the sensor capable of exerting a control action on the sensor to generate at the output a signal representing the ratio of the exhaust gases; the sensor comprising at least one reference chamber capable of receiving a specified percentage of oxygen, and being one of at least two types which differ in the way in which the reference chamber receives the oxygen; and the control device being characterized in that the controller comprises programmable control means and in that it comprises an operating unit capable of operating and programming the said control means to match the controller to the type of sensor to which it is connected.
Conveniently, the control device for the sensor, where the sensor comprises a diffusion chamber capable of receiving the exhaust gases and a first and a second electrolytic cell sensitive to oxygen ions, the first of these cells being controllable with respect to current, is characterized in that the said control means comprise a feedback circuit capable of regulating the current sent to the first cell in accordance with the difference between the percentages of oxygen present in the diffusion chamber and in the reference chamber; the said feedback circuit comprising a means of generating the said output signal in accordance with the current sent to the first cell; the controller being connected to a compensating resistance capable of compensating the losses of the said sent current, and comprising an acquisition circuit capable of acquiring the value of the compensating resistance; the operating unit being capable of correcting the output signal of the controller in accordance with the acquired value of the compensating resistance, to generate a corresponding output signal which is truly representative of the ratio of the exhaust gases and which is independent of possible changes in the compensating resistance.
The control device is thus capable of compensating the losses of the driving current, by always using the acquired value of the compensating resistance. This ensures the generation of an output signal which is truly representative of the ratio of the exhaust gases and is independent of possible changes in the compensating resistance.